WASHINGTON (CNN) -- The Constitution gives the president the exclusive power to nominate members of the Supreme Court. But it does not guarantee the political process will run smoothly for him or his nominee.

Supreme Court Justice David Souter was nominated by President George H.W. Bush.

Forces beyond his control often shape the tough choice the chief executive must make -- history left to the whim of a thousand "what-if" scenarios. Past confirmation battles offer some, but by no means definitive, guidance to who will be chosen and why.

Speculation over vacancies has always been present, but the ideological balance of the current court and recent partisanship of judicial confirmations has brought a perhaps unprecedented intensity to the process.

In the 220 years since the U.S. Constitution took effect, only 110 people have sat on the Supreme Court, an average of exactly one new justice every two years. The president may select whomever he wants -- the Constitution provides no prequalifications -- but it is no surprise every justice either was a lawyer or held a law degree. The current court is the first time all nine members were federal appeals court judges just prior to their elevation to the high court.

It has been more than three years since the latest justice took the bench, and a pent-up sense of anticipation has greeted President Obama's upcoming decision on a replacement for retiring Justice David Souter. It will be the first choice by a Democratic president in 15 years. In Washington, an unofficial parlor game of sorts has developed over who the next nominee will be.

"You don't have the sense the White House is itching for a fight. To the contrary, they are trying to consult very widely," said Thomas Goldstein, a leading appellate attorney and co-founder of scotusblog.com. "But on the other hand they are not shying away from people that they are being warned off of. They feel comfortable that they have the votes to get any reasonable candidate confirmed and they are just looking for the person the president determined to be the best justice of the Supreme Court."

When to leave: 'I'm getting old and coming apart'

Sitting justices are rarely pleased by the constant speculation about their retirement. In a 2003 CNN interview, Justice Sandra Day O'Connor admitted the thought of retirement had crossed her mind. "I mean, I'm getting up there in age so, of course, I think about should I or should I not," she said. "I don't know when."

But when pressed, O'Connor was abrupt. "I think that's enough, really. I just haven't made that decision." She retired two years later.

The late Chief Justice William Rehnquist said in a television interview a few years ago, "traditionally, Republican appointees have tended to retire during Republican administrations."

But often, age or ill health play the leading role in when a justice decides to step down, as was the case with two recent liberal lions of the court. Justice William Brennan was 84 when he resigned in 1990 after suffering a stroke, and Justice Thurgood Marshall was in declining health when he announced his 1991 retirement at age 82. Asked what was wrong with him, the feisty Marshall replied, "What's wrong with me? I'm old. I'm getting old and coming apart."

So as with all government leaders, the health problems of the justices inevitably become public fodder. Rehnquist's cancer diagnosis a few days before the 2004 election prompted instant, intense political speculation. He died less than a year later, still in office.

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John Paul Stevens, Ruth Bader Ginsburg, and O'Connor are all cancer survivors. O'Connor jokingly recalled what happened when she returned to the bench after medical treatment. "The worst was my public visibility, frankly. There was constant media converge: 'How does she look?' 'When is she gonna step down and give the president another vacancy on the court?' 'You know, she looks pale to me, I don't give her six months.' "

The political calendar also plays a role. Justices have traditionally tended not to step down during presidential election years, to avoid making the replacement and the institution itself a campaign issue. The last time it happened was 1956, when Brennan was nominated by President Dwight Eisenhower, who hoped his choice of a northeastern Catholic would help win re-election votes from that important political constituency. The conservative Eisenhower later regretted the decision, as Brennan went on to establish an influential liberal legacy in his 34 years on the bench.

The Senate's political makeup also plays a key factor. Democrats currently have a clear majority, and Obama's pick is expected to be confirmed rather easily, barring some unexpected personal or political controversy.

"The process has become more partisan," said David Yalof, author of "Pursuit of Justices," a book about recent court selections. "In part that's because we've anticipated a vacancy, and because the political stakes, as usual, are so high."

And finally, time of year can often make the confirmation process easier. Souter announced May 1 he would retire, five months before the next court term begins. Former Justices Byron White and Harry Blackmun stepped down in March and April respectively, to give President Bill Clinton ample time to find a nominee and win confirmation.

But recent justices Potter Stewart, Warren Burger, Marshall and Brennan all stepped down in late June or early July, at the effective end of the court's term. That left only about three months to confirm a candidate before the start of the fall session.

Factors of gender, race and age

Diversity is not a word that describes the Supreme Court's makeup over most of its existence. Only two women justices have served, the first in 1981, and only two African-Americans, the first in 1967. No Hispanic- or Asian-Americans have been named, nor have any openly homosexual or disabled justices. Only about nine Catholics have served, and seven Jews.

With the advent in the 1920s of open Senate hearings of court nominees, public and press scrutiny of the process grew. Tremendous social and political changes in the country were reflected in a slow push to diversify the court's makeup. Since then, seats unofficially have been routinely set aside for individuals of certain backgrounds.

The first Jewish justice was Louis Brandeis, whose bitter 1916 confirmation was marked by underlying anti-Semitism. When Benjamin Cardozo joined him in 1932, a "Jewish seat" was established. Cardozo was replaced by Felix Frankfurter, who was replaced by Arthur Goldberg, who was replaced by Abe Fortas. Currently, two Jewish justices sit on the bench, Ginsburg and Stephen Breyer.

A similar precedent was created when Marshall became the first person of color to sit on the court. When he retired, Clarence Thomas took Marshall's seat, although President George H.W. Bush considered a wide variety of candidates before settling on the Georgia native.

Many legal experts say Obama could make his first court pick a Hispanic, thereby breaking new historical ground and securing political outreach to a vital constituency. But a far more pressing factor for the president with this pick will be gender diversity.

Some political analysts think a Hispanic pick also could neutralize resistance from hard-core conservatives who would soften their attacks for fear of being labeled culturally insensitive.

"There is only one thing that is essential for this pick, probably: That she be a woman," said Goldstein. "Beyond that, the candidate [being] Hispanic would certainly be a plus because it would add still more to the diversity of the Supreme Court. It would be historic in that sense but it's not going to be determinative."

Ginsburg currently is the only woman on the high court.

Age is another consideration. The oldest person named to the court was 65-year-old Horace Lurton in 1910. Relatively younger candidates -- those in their 40s or early 50s -- may receive greater consideration from the president, since they would be likely to spend more years on the bench and perhaps have a greater judicial impact. That impact would potentially enhance a presidential legacy.

One conservative with another

Ideology is hard to characterize, particularly in judicial candidates who presumably are not disposed to view the law with a strictly political lens. The term "reliably conservative" or "reliably liberal" may be the best a president can hope for when considering candidates for the Supreme Court.

But such choices can backfire. Take the case of Earl Warren. He was California's governor and a rival of Eisenhower in the presidential primaries of 1952. Warren eventually dropped out of the race and threw his support to the retired general. As political payback and to court Western conservatives, Ike selected Warren as chief justice in 1953, despite Warren's lack of judicial experience.

Warren's once-conservative views soon moderated, and under his leadership, the Supreme Court established a lasting liberal record, particularly in civil rights, due process and individual protections. Eisenhower later called the selection "the biggest damn-fool mistake I ever made."

Two words have recently entered the political lexicon: to "Souter" and to "Bork"-- named after Supreme Court choices, one successfully nominated, the other a political casualty. They have come to represent how judicial candidacies can take on a life of their own.

Conservatives in 1990 cheered the retirement of Brennan, a liberal thorn to them for three-and-a-half decades. The choice of Souter to replace Brennan came as a complete surprise. His legal record was sparse, and few knew where he stood on key issues. He was labeled the "stealth nominee" and there was bipartisan hand-wringing over what he would do on the bench.

The country soon found out, as Souter has carved a mostly moderate-liberal position. That evolving centrist stance outraged conservatives, and they vow never to be caught off guard with another nominee. To that end, extensive, almost obsessive vetting of potential candidates has now become common. Groups of all political stripes compile massive dossiers, ready to use the information as political weapons. To "Souter" has come to mean to name a candidate without knowing much about him.

Robert Bork had the opposite problem. His paper trail was extensive. He was an outspoken and prolific jurist with a clear, unapologetic conservative record. He would have succeeded Lewis Powell, a centrist and the court's key swing vote.

Liberals realized a Bork victory would immediately tip the bench rightward. His confirmation hearing was the most bitter Washington had seen in years, and he was ultimately rejected by the Senate 58-42, the largest margin of defeat ever for a court nominee. To "Bork" has since come to mean to wage a partisan fight against judicial nominees.

Unlike most past presidents, Obama has called upon his experience as a constitutional law professor to spell out the kind of justice he'd like to see, using "empathy" as a key barometer.

"I will seek somebody with a sharp and independent mind and a record of excellence and integrity," he said May 1. "I will seek someone who understands that justice isn't about some abstract legal theory or footnote in a case book; it is also about how our laws affect the daily realities of people's lives -- whether they can make a living and care for their families; whether they feel safe in their homes and welcome in their own nation."

He has cited Ginsburg and Breyer as two justices he admires. Not surprisingly, they are the only two on the current court named by a Democratic president.

Length of confirmation: Anything goes

Democrats are in charge of the White House and Senate; they control the nomination process. But Republicans still retain power to influence events, and have at their disposal an old legislative maneuver, the filibuster, to delay a vote on any federal court nominees.

But there is no indication the GOP leadership has the votes or the political stomach to launch such a fight now.

The amount of time taken to prepare for and hold confirmation hearings has also become a political issue. Obama hopes to have the process completed and a full Senate vote on the nominee by the August 8 recess. Some Republicans, however, fear Democrats may try to speed up the process and hold hearings as early as possible, thereby blunting efforts by conservatives to rally opposition.

Congressional sources suggested the hearings themselves could also be lengthy, a break with recent tradition. Seven of the past eight nominees spent around a week in hearings. The notable exception was Clarence Thomas.

Thomas' initial hearings were partisan, and he avoided questions on his personal views on issues, especially abortion. The Judiciary Committee deadlocked over whether to confirm him, and sent his nomination to the full Senate without a recommendation.

Two days before a vote, Anita Hill, a former co-worker of Thomas in the Reagan administration, accused him of sexual harassment. New hearings were scheduled, marked by graphic descriptions of what Hill called Thomas' advances, and his dramatic denials. He labeled the hearings "a high-tech lynching for an uppity black." They ended without resolving the matter, and Thomas was eventually confirmed.

"How long the hearings last really depends on who the nominee is," said David Garrow, a Pulitzer Prize-winning historian and Supreme Court expert. "If he or she falls into what I call the 'Breyer category,' where nobody could find anything objectionable to say about this guy, hearings will be a breeze. If he falls into perhaps the 'Bork category,' anything goes."